Multistep Relaxation Pathway from Glass to Crystal Development in Poly( <scp>l</scp> -lactide)

The thermal transitions of amorphous poly-(l-lactide) (PLLA) were investigated by combining standard calorimetry (DSC), fast scanning calorimetry (FSC), and infrared spectroscopy. By following the enthalpic evolution, the glass transition temperature (<i>T</i> _g) change, and the chains' conformational behavior of PLLA during sub-<i>T</i> _g and supra-<i>T</i> _g annealing isotherms, a transition was identified in the supercooled liquid (SCL) phasethe so-called SCL transformationoccurring before crystallization. SCL transformation kinetics was assessed and introduced into the relaxation map of amorphous PLLA, revealing a hierarchical pathway. The SCL transformation allows the unstable PLLA-quenched liquid to relax toward a metastable SCL with a lower free energy. This occurs through conformational rearrangement of a small fraction of <i>gg</i> conformers into the more stable <i>gt</i> form. Ultimately, SCL kinetics was compared to PLLA liquid dynamics, providing a suggestive scenario where the SCL transformation is mediated by both α-relaxation and non-α relaxation mechanismsthe Slow Arrhenius Process (SAP).